Method for operating rotary hearth type reducing furnace and rotary hearth type reducing furnace facilities

ABSTRACT

The present invention provides a method of operation and a facility for the same suppressing the generation of dioxins in the combustion exhaust gas and efficiently reclaiming heat from high temperature combustion exhaust gas when firing and reducing fines of chromium ore, iron ore, or other ore or pellets formed from dust sludge, etc. containing iron oxide or other metal oxides generated in the metal industry in a reducing rotary hearth furnace. This treats the combustion gas generated in the reducing rotary hearth furnace to make the temperature of the gas 800° C. or higher for at least a certain time, to make the concentration of the carbon monoxide not more than 200 ppm in terms of volume ratio and to achieve a sufficiently well developed turbulent state at least at one of the inside of the exhaust gas outlet duct and the vicinity of the exhaust gas outlet duct for at least a certain time, then rapidly cooling the gas. Further, it controls the total number of moles of zinc and lead, the total number of moles of alkali metals, and the total number of moles of halogen elements contained in the combustion exhaust gas to a predetermined ratio.

TECHNICAL FIELD

[0001] The present invention relates to a method of operation whichsuppresses the production of dioxins in combustion gas in a reducingfurnace, efficiently exchanges heat with and reclaims heat from hightemperature combustion exhaust gas, and suitably treats the combustionexhaust gas when heating, firing, and reducing pellets or other shapedarticles containing metal oxides in a rotary hearth reducing furnace orother reducing furnace and a facility for the same.

BACKGROUND ART

[0002] There are various types of processes for producing reduced ironor iron alloy. There are the Waelz kiln method of heating and reducingfeedstock inside a rotary kiln while tumbling with carbon and a reducingagent, the rotary hearth method of firing and reducing feedstock in areducing rotary hearth furnace, etc. Among these, as a process with ahigh productivity, operation is being performed by the rotary hearthmethod such as shown in Japanese Unexamined Patent Publication (Kokai)No. 200054034. The rotary hearth method is a process based on heatingreducing furnace of a type reducing pellets or other shaped articles ofa powder feedstock charged on the hearth and moving through a heatingzone, reducing zone, and ejector by rotating a disk-shaped hearth ofrefractories with a cutaway center at a constant speed on rails under afixed ceiling and side walls made of refractories (hereinafter called a“rotary furnace”) and is used for the reduction of metal oxides. Thediameter of the hearth of the rotary furnace is 10 to 50 meters, and thewidth of the hearth is 2 to 6 meters.

[0003] As the feedstock, fine ore or metal oxide dust or other metaloxides and carbon serving as a reducing agent are used. In theproduction of reduced iron, pellet feed ore or other particulate ironore etc. is used. Carbon is used as the reducing agent, but an agentwith a high percentage of carbon not volatilizing up to about 1100° C.at which a reduction reaction takes place, is preferable (hereinafterthis carbon is called fixed carbon). For such a carbon source, fine cokeor anthracite is preferred.

[0004] The powder containing the metal oxides of the feedstock is mixedwith an amount of reducing agent containing carbon required for reducingthe metal oxides, then is pelletized to form green pellets which arethen fed in layers on the hearth of the rotary furnace. The greenpellets are spread on the hearth of the rotary furnace. In a rotaryfurnace, a circular hearth with a cutaway center rotates under arefractory ceiling and side walls. The pellets on the hearth are movedthrough the several parts of the furnace and heated rapidly. They arefired for 5 minutes to 20 minutes at a high temperature of around 1300°C., whereby the metal oxides are reduced by the carbon in the pellets,and metal is produced. In the rotary furnace, since the green pelletsare placed stationarily on the hearth, there is the advantage that thegreen pellets are resistant to crumbling in the furnace. As a result,there are the strong points that there is no problem of the powderizedfeedstock sticking on the refractories in the rotary furnace and theyield of pellets is high. Further, this process is being used in manycases in recent years since the productivity is high and it is possibleto use an inexpensive coal-based reducing agent or powder feedstock.

[0005] Further, the rotary hearth method is also effective for treatmentto reduce and remove impurities from the ironmaking dust generated froma blast furnace, converter, or electric furnace, or scale or thickenersludge generated from the rolling process. It is also used as a dusttreatment process and is a process effective for recycling of metalresources. The fact that when reducing dust, the rotary hearth method ispossible to remove zinc, lead, alkali metals, halogens, and otherimpurities is also an advantage of the rotary hearth method. This is thereason why this is a particularly effective process for recycling ofdust etc. generated in the ironmaking industry.

[0006] The operation in the rotary hearth method can be summarized asfollows. First, the feedstock, that is, the metal oxides such as thefine ore or the dust or sludge are mixed well with an amount ofcarbonaceous reducing agent required for the reduction of the oxides,then pelletized. There are several methods of pelletization. Forexample, green pellets of 5 to 20 mm size are produced by a pan typepelletizer or other types of pelletizer to have a moisture content of 8to 15 wt %. These green pellets are fed onto the rotary hearth in layersand spread over the hearth. The raw pellets spread over the hearth arerapidly heated in the furnace and fired for 5 to 30 minutes at a hightemperature of around 1300° C. At this time, the metal oxides arereduced by the carbon of the reducing agent mixed in with the greenpellets, thus metal is produced. The amount of fixed carbon in thereducing agent is substantially determined by the amount of oxygenbonded with the metals to be reduced. The metallization ratio afterreduction differs depending on the metal to be reduced, but in the caseof iron, nickel, or manganese, it is at least 90% and even with the caseof chromium, which is hard to be reduced, it is at least 40%.

[0007] As explained above, the rotary hearth method is an efficient,good process for reducing the metal oxides at a rotary hearth reducingfurnace, in particular the dust, scale, sludge, etc. generated from theindustries of refining or processing metals, to obtain reduced metal.When using dust or other by-products of the metal industry as feedstock,this is an effective means in recycling.

[0008] The fine ore used as part of the feedstock also includes chlorineor another impurities. In particular the by-products in the metalindustry include machine oil, organic matter in water, chemicals withchlorine agents, resin powder, and other impurities. For example, thesludge deposited in a scale pit, which is generated in the process ofrolling a steel material, contains 1 to 5 mass % of machine oil.Further, blast furnace dust contains 0.1 to 0.3 mass % of chlorine.

[0009] Further, electric furnace dust used for reduction in a reducingrotary hearth furnace also contains chlorine and oil, and in addition,it contains dioxins themselves. Most of these impurities burn orvaporize inside the rotary furnace and are discharged from the furnace.During this process, the organic matter burns and forms carbon dioxideor water vapor. If the combustion is incomplete, however, sometimessoot, carbon, unburned benzene, etc. may be contained in the combustiongas. Further, the chlorine ingredient forms chlorine gas or hydrogenchloride gas or salts such as sodium chloride, zinc chloride, etc. Alarge concentration of each of the materials is discharged from thefurnace together with the combustion gas. These organic substances andchlorine react in the combustion gas and generate dioxins, though theamounts are small. In particular, the generation of dioxins increaseswhen the combustion in the furnace is incomplete. When the concentrationof the carbon monoxide in the combustion gas is high, the amount ofdioxins generated increases due to the reaction of the chlorine andbenzene or a synthesis reaction from the gas phase.

[0010] With the conventional facility configuration or method ofoperation, however, there was not sufficient knowledge and operation wasnot necessarily performed from the viewpoint of reduction of thegeneration of dioxins. In conventional operation, the main objective wasjust simply to supply heat to the reduction reaction. Further, theexhaust gas treatment initially was mainly to prevent deposition of dustin the path of the combustion gas treatment or the reclamation of wasteheat. In fact, the methods of reduction of dioxins were not sufficientlyreflected in the design of the facilities. In subsequent investigations,it was found that in rotary furnaces since the combustion temperatureinside the furnace is higher, the amount of generation of the dioxins issmaller and the environmental load is smaller compared with othercombustion processes, but dioxins cannot be totally eliminated in thecombustion gas, that is, sometimes, they are present in a concentrationof as much as 1 to 5 ng-TEQ/Nm³.

[0011] That is, in the operation of a reducing furnace such as areducing furnace of the rotary hearth method according to the prior art,no good method of operation for effectively controlling the productionof dioxins has yet been discovered.

[0012] Further, as explained above, a rotary furnace, Waelz kiln, orother reducing furnace generates high temperature combustion exhaust gascontaining a large amount of carbon dioxide and water vapor. Thiscombustion exhaust gas is discharged at a rate of 2000 to 3000 Nm3 perton of feedstock. This exhaust gas contains dust generated from theinside of the furnace, passes through the exhaust gas duct, is cooled bythe method of spraying water or by other methods in the exhaust gastreatment apparatus, then is cleaned of dust and emitted into theatmosphere. As explained above, the rotary furnace method is a processwith a relatively large amount of generation of dust since the zinc,lead, chlorine, and other impurities are removed by vaporization duringthe reducing reaction of oxides.

[0013] In this way, in the operation of a rotary hearth reducingfurnace, rotary kiln, or other metal reducing furnace, a large amount ofcombustion exhaust gas containing a large amount of dust is generated.The sensible heat held by the exhaust gas corresponds to about 30% ofthe total input energy. Reclamation of the heat of the combustionexhaust gas plays an important role in operation with a good heatefficiency.

[0014] However, when trying to reclaim waste heat of high temperaturecombustion exhaust gas, there were the problems that the dust stronglystuck to the heat transmission surfaces of the waste heat boiler or heatexchanger, or corroded the metal of these surfaces. As a result, forexample in the exhaust gas treatment method in a reducing furnace, asshown in Japanese Unexamined Patent Publication (Kokai) No. 2000-169906,even in the prior art, a method of suitably controlling the temperatureof the boiler or heat exchanger was adopted to prevent the deposition ofdust. This method is an effective means for treatment of exhaust gas,but depending on the ingredients of the dust, there was the problem thatdust deposited on the inside surfaces of the heat exchanger and the pathof the exhaust gas was clogged within 2 weeks to one month or so. Inparticular, if lowering the melting point of the dust, the adhesionpower of the dust becomes stronger and the problem becomes greater.

[0015] The dust generated from a rotary furnace etc. includes not onlythe carried-over substances of feedstock such as iron oxide, but alsolarge amounts of alkali metals, zinc, lead, or other volatile metals andcationic substances such as chlorine. At the portion of the exhaust gasoutlet duct of 600 to 1100° C. or so, the ingredients of the dust arepresent as vapor. This starts to deposit as a liquid along with a dropin the temperature of the exhaust gas. The ingredients of dust scatteredas solids and the liquids form an emulsion with a high viscosity. Thissticks to the path of the exhaust gas, so the path becomes narrower andthe problem of easy blockage at that portion arises. That is, wheninstalling a heat exchanger so as to reclaim waste heat, the path of theexhaust gas becomes narrower at that portion and blockage easily takesplace. A liquid of an alkali metal salt is strongly cohesive and therewas also the problem of metal corrosion at the portion where thisemulsion stuck.

[0016] The method of the above Japanese Unexamined Patent Publication(Kokai) NO. 2000-169906 is a technique effective for the prevention ofdeposition of dust. In particular, when there is a large amount ofalkali metal salts and zinc compounds, deposition could not besufficiently prevented. In this way, in the prior art, sufficientattention has not been paid to the control of the ingredients of dust inthe exhaust gas and the problem of clogging of a heat exchanger forreclamation of the heat of the exhaust gas has not been sufficientlysolved.

[0017] Due to this, a technology has been sought which solves theabove-mentioned problems and decreases the generation of dioxins incombustion exhaust gas discharged from a rotary furnace, rotary kiln, orother metal reducing furnace and efficiently reclaims sensible heat heldby this exhaust gas.

DISCLOSURE OF THE INVENTION

[0018] The present invention was made to solve the above problems andhas the following as its gist:

[0019] (1) A method of operation of a reducing rotary hearth furnacecharacterized by making the combustion gas generated inside a reducingrotary hearth furnace a temperature of 800° C. or higher at least at oneof the inside of an exhaust gas outlet duct of the reducing furnace andthe vicinity of the exhaust gas outlet duct, then rapidly cooling it,collecting dust, then discharging the result.

[0020] (2) A method of operation of a reducing rotary hearth furnacecharacterized by making the combustion gas generated inside a reducingrotary hearth furnace a temperature of 800° C. or higher at least at oneof the inside of an exhaust gas outlet duct and the vicinity of theexhaust gas outlet duct, making a concentration of carbon monoxide notmore than 200 ppm by volume ratio, making the gas a well developedturbulent state, then rapidly cooling the combustion gas.

[0021] (3) A method of operation of a reducing rotary hearth furnacecharacterized by making the combustion gas generated inside a reducingrotary hearth furnace a temperature of 800° C. or higher for at least0.9 second at least at one of the inside of an exhaust gas outlet ductand the vicinity of the exhaust gas outlet duct, making a concentrationof carbon monoxide not more than 200 ppm by volume ratio, making the gasa turbulent state of a Reynolds number of at least 30,000, then rapidlycooling the combustion gas.

[0022] (4) A method of operation of a reducing rotary hearth furnacecharacterized by making the combustion gas generated inside a reducingrotary furnace a temperature of 800° C. or higher for at least 0.6second at least at one of the inside of an exhaust gas outlet duct andthe vicinity of the exhaust gas outlet duct, making a concentration ofcarbon monoxide not more than 60 ppm by volume ratio, making the gas aturbulent state of a Reynolds number of at least 50,000, then rapidlycooling the combustion gas.

[0023] (5) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (4), characterized in that the combustiongas generated in the furnace has a temperature inside the furnace of atleast 1200° C. and a molar ratio of carbon monoxide to carbon dioxide ofat least 0.1.

[0024] (6) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (5), characterized in that the combustiongas generated inside is cooled from 800° C. or higher to not more than300° C. within 6 seconds.

[0025] (7) A method of operation of a reducing rotary hearth furnace asset forth in (6), characterized in that the combustion gas generatedinside is cooled in an exhaust gas treatment apparatus provided with anyone of a waste heat boiler, water sprayer, and heat exchanger with airalone or in combination.

[0026] (8) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (7), characterized in that the totalnumber of moles A of zinc and lead, the total number of moles B ofsodium and potassium, and the total number of moles C of chlorine andfluorine contained in the dust of the combustion gas generated insidesatisfy the relationship of (C−B)/A<0.36.

[0027] (9) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (8), characterized in that the percentcontent of compounds of sodium or potassium and chlorine or fluorinecontained in the dust of the combustion gas generated inside is not morethan 35 mass %.

[0028] (10) A method of operation of a rotary hearth reducing furnace asset forth in (8) or (9), characterized in that the temperature of thecombustion gas generated inside is cooled from 800° C. or higher to 550°C. or lower within 5 seconds.

[0029] (11) A method of operation of reducing rotary furnace wherein acombustion exhaust gas is cooled in a gas treatment apparatus providedwith at least a preheating type heat exchanger preheating air by heatexchanging, said method characterized in that the total number of molesA of zinc and lead, the total number of moles B of sodium and potassium,and the total number of moles of C of chlorine and fluorine contained inthe dust of the combustion gas generated inside satisfy the relationshipof (C−B)/A<0.36.

[0030] (12) A method of operation of a reducing rotary hearth furnace asset forth in (11), characterized in that the content ratio of compoundsof sodium or potassium and chlorine or fluorine contained in the dust ofthe combustion gas generated inside is not more than 35 mass %.

[0031] (13) A method of operation of a reducing rotary hearth furnace asset forth in (11) or (12), characterized in that the temperature of theexhaust gas at the inlet of the exhaust gas outlet duct of the reducingfurnace is 800° C. or higher and the temperature of the exhaust gas atthe inlet of the air preheating type heat exchanger of the exhaust gastreatment apparatus is 550° C. or lower.

[0032] (14) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (13), characterized by cooling thetemperature of the exhaust gas from 800° C. or higher to 550° C. orlower within 5 seconds.

[0033] (15) A method of operation of a reducing rotary hearth furnacefor firing and reducing a feedstock obtained by shaping a powdercontaining a metal oxide and carbon and cooling the combustion exhaustgas in a reducing rotary hearth furnace having an exhaust gas treatmentapparatus provided with at least an air preheating type heat exchanger,the method of operation of a rotary hearth reducing furnacecharacterized in that the relationship of (0.8C′−0.7B′)/A′<0.36 standsamong the total number of moles A′ of zinc and lead, the total number ofmoles 5′ of sodium and potassium, and the total number of moles C′ ofchlorine and fluorine in the feedstock, the temperature of the exhaustgas at the exhaust gas outlet duct of the reducing furnace is made 800°C. or higher, and the temperature of the exhaust gas at the inlet of theair preheating type heat exchanger is made at least 550° C.

[0034] (16) A method of operation of a reducing rotary hearth furnace asset forth in any one of (1) to (15), characterized in that the cooledcombustion gas generated inside the furnace is cleaned of dust by a bagfilter of an exhaust gas treatment apparatus.

[0035] (17) A reducing rotary hearth furnace facility characterized bybeing provided with a reducing furnace for a metal oxide having a rotaryhearth and an exhaust gas treatment apparatus comprised of a coolercomprised of a waste heat boiler, water sprayer, or air preheating heatexchanger alone or in combination and a dust collector and by connectingthe reducing furnace and the exhaust gas treatment apparatus by a ducthaving a length of at least 0.9 second converted to time of passage ofthe combustion gas generated inside the reducing furnace and a productof the inside diameter and flow rate of gas inside at least 7.2 m²/sec.

[0036] (18) A reducing rotary hearth furnace facility characterized bybeing provided with a reducing furnace for a metal oxide having a rotaryhearth and an exhaust gas treatment apparatus comprised of a coolercomprised of a waste heat boiler, water sprayer, or air preheating heatexchanger alone or in combination and a dust collector, and byconnecting the reducing furnace and the exhaust gas treatment apparatusby a duct having a length of at least 0.6 second converted to time ofpassage of the combustion gas generated inside the reducing furnace anda product of the inside diameter and flow rate of gas inside at least 12m²/sec.

[0037] (19) A reducing rotary hearth furnace facility as set forth in(17) or (18), characterized in that the cooler in the exhaust gastreatment apparatus has an inside capacity of the portion cooling thecombustion gas from 800° C. or higher to not higher than 300° C. or notmore than six times the amount of flow of combustion gas per second.

[0038] (20) A reducing rotary hearth furnace facility characterized bybeing provided with a reducing furnace for a metal oxide having a rotaryhearth and an exhaust gas treatment apparatus comprised of a coolercomprised of a waste heat boiler, water sprayer, or air preheating heatexchanger alone or in combination and a dust collector and by beingprovided with a means for removing dust deposited on the air preheatingheat exchanger of the exhaust gas treatment apparatus of at least one ofan impact/vibration imparting device, gas blower, and scraper.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a view of an example of the configuration of a reducingrotary hearth furnace facility having a reducing rotary hearth furnaceand an exhaust gas treatment apparatus for working the presentinvention.

[0040]FIG. 2 is a view of a cross-section of a reducing rotary hearthfurnace.

[0041]FIG. 3 is a view of a reducing rotary hearth furnace laid out inthe circumferential direction.

[0042]FIG. 4 is a view of the relationship between the holding time of asuitable combustion gas state and the concentration of dioxins in thecombustion gas.

[0043]FIG. 5 is a view of the relationship between the time for coolingcombustion exhaust gas from 800° C. to 300° C. and the concentration ofdioxins in the combustion exhaust gas.

BEST MODE FOR CARRYING OUT THE INVENTION

[0044] The present invention will be explained with reference to areducing furnace of a rotary hearth type shown in FIG. 1 as anembodiment of a reducing furnace. The process shown in FIG. 1 is mainlycomprised of a reducing rotary hearth furnace 1, a waste heat boiler 3,a water sprayer 4, a heat exchanger (air preheater) 5, and a dustcollector 7. The views showing the structure of the rotary furnace areFIG. 2 and FIG. 3. A cross-section view of a reducing furnace is shownin FIG. 2. Further, a view of a disk-shaped hearth with a center cutaway inside the furnace laid out in the rotational direction is shown inFIG. 3.

[0045] The structure of the reducing furnace 1, as shown in thecross-section of FIG. 2, consists of a hearth 12 moving and rotating onwheels 13 under a fixed ceiling 10 and walls 11 made of refractories.The walls 11 are provided with burners 14. These blow fuel and generatehigh temperature combustion gas by flames 15. Green pellets 16 comprisedof metal oxide powder and carbon powder are spread over the hearth 12.Details of the heating and reduction of the green pellets 16 and otheroperations will be explained by FIG. 3 of the rotary furnace 1 laid outin the circumferential direction.

[0046] The green pellets as a feedstock are formed by mixing fine ore orby-products of ironmaking such as converter gas dust or scale etc. andpowder of reducing agent containing carbon and by shaping the mixturesuch as powder coke etc. The green pellets of this feedstock are spreadover the hearth 12 from a feedstock inlet 17. Normally, the thickness isone to three layers. The hearth 12 moves and rotates. Along with thisthe green pellets 16, first, enter the heating zone 18. The ceiling ofthe heating zone 18 is connected to an exhaust gas outlet duct 2. Thegreen pellets 16 heated there enter the reducing zone 19 with a higherconcentration of carbon monoxide and hydrogen and a high temperature.There, the metal oxides inside the green pellets 16 react with thecarbon and are reduced to metals. Next, the reduced green pellets 16 aredischarged from the furnace by an discharging screw 21 of the reducedpellet ejector 20. The combustion gas flows in a direction opposite tothe pellets, that is, from the high temperature reducing zone 19 to therelatively low temperature heating zone 18, where it is dischargedthrough the exhaust gas outlet duct 2 to the outside of the furnace.Note that while not necessarily required, partition plates 22 aresometimes set for dividing the zones efficiently.

[0047] The combustion gas inside the furnace is the result of combustionof mixture of the fuel and air from burners 14 arranged at the furnaceside walls from the reducing zone 19 to the heating zone 18 and themostly carbon monoxide gas generated from the green pellets. Thiscombustion gas flows from the reducing zone 19 toward the heating zone18. Next, the gas is discharged from the exhaust gas outlet duct 2 atthe ceiling of the heating zone 18 to the outside of the rotary furnace1.

[0048] First, in the reducing zone 19, it is necessary to maintain areducing atmosphere sufficient for reducing the green pellets. The metaloxides covered by the reduction in the rotary hearth method includeiron, nickel, chromium, lead, zinc, and manganese. From the reducibilityof these metals, the reducing zone 19 has to have a temperature of atleast 1200° C. and a molar ratio of carbon monoxide to carbon dioxide ofat least 0.1. Under these conditions, reduction of the green pellets 16proceeds rapidly.

[0049] Further, if the temperature is over 1200° C., even the organicmatter and chlorine mixed in with the green pellets 16 are broken downinto carbon monoxide, carbon dioxide, water vapor, hydrogen, andhydrogen chloride and the amount of generation of dioxins is relativelykept down.

[0050] Next, the combustion gas inside the furnace flows to the heatingzone 18. Here, the green pellets 16 are heated to a temperature of about1100° C., but are not reduced. The temperature of the combustion gashere is made 1100 to 1200° C. at the portion near the reducing zone 19and made 800 to 1100° C. at the portion near the exhaust gas outlet duct2. Further, the temperature at the portion of the exhaust gas outletduct 2 connected with the waste heat boiler 3 is made 800° C. or higher.Further, since no reduction takes place in the heating zone 18, theatmosphere of the combustion gas can be low in reducibility.

[0051] Therefore, at least the portion close to the exhaust gas outletduct 2 contains excessive oxygen and is preferably kept lowconcentration of carbon monoxide.

[0052] Note that in the present invention, the portion close to theexhaust gas outlet duct 2 of the heating zone 18 in the reducing furnaceand the portion of the exhaust gas duct 2 connected with the waste heatboiler 3 are together called the “vicinity of the exhaust gas outletduct”.

[0053] As the composition of the combustion gas at the vicinity of theexhaust gas duct and at the exhaust gas duct 2, in order to keep theconcentration of carbon monoxide not more than 200 ppm by volume ratio,the concentration of oxygen is made 0.5 vol % or more at the portion ofthe exhaust gas outlet duct 2 connected with the waste heat boiler 3.Part of the portion corresponding to the conditions described isincluded inside the rotary furnace 1 as well, however from the viewpointof control of the composition of the combustion gas, it is preferably inthe middle of the exhaust gas outlet duct 2. Further, in the example ofFIG. 1, a waste heat boiler 3 reclaiming heat is connected to theexhaust gas outlet duct 2, however if it is possible to cool the gasrapidly, it is also possible to connect other types of gas cooler orheat reclaimer. Further, from the viewpoint of the problems inmanagement of the combustion gas in the flue and safety and the furtherreduction of dioxins, the concentration of carbon monoxide is morepreferably reduced to not more than 60 ppm by volume ratio. Further, inaddition to these conditions, from the viewpoint of improvement ofmixing of the gases, if making the state of flow of the exhaust gas awell developed state of turbulence, it is possible to reduce thedioxins. Note that the method of development of turbulence will beexplained later.

[0054] After then, the combustion gas is cooled to not higher than 300°C. by the waste heat boiler 3, water sprayer 4, and air preheating heatexchanger 5. The time required for cooling is preferably not longer than6 seconds. The cooled combustion gas passes through the duct 6 beforedust collection, is cleaned of dust at the dust collector 7, is drawn inby the induction fan 8, then is dispersed from the smokestack 9 to theatmosphere. Note that the dust collector 7 used is a bag filter type.That is, the combustion gas of the rotary furnace is raised to 800° C.or higher and made to completely burn at least at one of the inside ofthe exhaust gas outlet duct and the vicinity of the exhaust gas outletduct, then is rapidly cooled and cleaned of dust so as to suppress thegeneration of dioxins and prevent the deposition of dust.

[0055] The inventors investigated the operating conditions forsuppressing the generation of dioxins in the combustion gas based on theoperating characteristics of a rotary furnace. For this purpose, theyconducted several experiments and drew the conclusion that it issufficient to treat the combustion gas under the above operatingconditions. Note that the content of the experiments will be explainedin detail below.

[0056] The inventors found that to suppress the generation of dioxins,control of the temperature and gas compositions is important duringtreating the combustion gas from the heating zone 18 of the rotaryfurnace on. Dioxins are generated in large amounts in a state ofincomplete combustion at an ambient temperature of 300 to 600° C. Thatis, several complicated organic molecules in the incompletely combustedgas react with chlorine to form dioxins at 300 to 600° C. Therefore, toeliminate the presence of complicated organic molecules such as benzenein this temperature range, the combustion gas in the reducing zone 19which had contained large amounts of carbon monoxide is made tocompletely burn at a high temperature of 800° C. or higher at theportion from the heating zone 18 to the exhaust gas outlet duct 12, thatis, inside the exhaust gas duct 2 and the vicinity of the exhaust gasduct.

[0057] Further, if trying to completely burn the gas at a hightemperature, the combustion gas is preferably mixed well in a state ofexcess oxygen. Therefore, the inventors studied the conditions ofcombustion gas under turbulent state to prevent the generation ofdioxins since giving sufficiently well developed turbulence to thecombustion gas would be an effective means for mixing the gas. FIG. 4shows the results of the experiments. FIG. 4 shows the suitableconditions, that is, the relationship between the holding time where thetemperature of the combustion gas is 800° C. or higher and theconcentration of carbon monoxide is 200 ppm or less by volume ratio inthe state of turbulence and the concentration of dioxins in the exhaustgas. As shown in FIG. 4, if the time when the conditions for suppressionof dioxins are met, that is, the temperature of the combustion gas ismore than 800° C. or higher and the concentration of carbon monoxide is200 ppm or less in volume ratio, is 0.9 second or longer, theconcentration of dioxins is a good one of 0.1 ng-TEQ/Nm3 or less. Thatis, it was learned that if sufficiently developing turbulence andsimultaneously continuing the suitable state of combustion gas for 0.9second, the above complicated organic molecules are no longer presentand the conditions for suppression of the generation of dioxins can besatisfied.

[0058] To create suitable turbulence for mixing combustion gas, it issufficient to make the indicator for evaluation of the state of flow influid dynamics, that is, the Reynolds number, larger. In general, whenthe Reynolds number exceeds 2300, the flow becomes turbulent. However,in the present invention, a good state for mixing the combustion gas,that is, sufficiently well developed turbulence, is necessary, so alarger Reynolds number is required. In experiments by the inventors, itwas found that under a state of turbulence with a Reynolds number of30,000 or larger, a uniform state of combustion gas, which is aimed at,is obtained. Therefore, a condition of the present invention is that thecombustion gas have a temperature of 800° C. or higher and have aReynolds number of 30,000 or larger at the portion where theconcentration of the carbon monoxide is 200 ppm or less by volume ratio.

[0059] Further, to reduce the dioxins more, a lower concentration ofcarbon monoxide and a higher Reynolds number are sought. The inventorsinvestigated and as result found that to make the concentration ofdioxins in the exhaust gas a better state of 0.06 ng-TEQ/Nm³ or lower,the concentration of carbon monoxide is preferably made a volume ratioof 60 ppm or less and a Reynolds number of 50,000 or larger. In thisstate, the state of mixing is extremely good, so a sufficient effect isgiven even if the time of continuation of this state is 0.6 second. Ifthis condition is maintained, an extremely good result of theconcentration of the dioxins in the exhaust gas being 0.02 to 0.06ng-TEQ/Nm³ is obtained.

[0060] Note that the Reynolds number is a dimensionless number and isobtained by dividing the product of the gas density, gas flow rate, andrepresentative length by the gas viscosity. The Reynolds number is anindicator expressing the state of flow. When it is over 2300, the higherthe Reynolds number, the more turbulent the flow. The representativelength in calculation of the Reynolds number in the present inventionused is the width of the hearth 12 or the diameter of the exhaust gasoutlet duct 2. At the temperature of the combustion gas under theconditions of the present invention, that is, 800 to 1200° C., thedensity of the combustion gas is about 0.25 to 0.4 kg/m³ and theviscosity is 4×10-⁵ to 6×10-⁵ kg/ms. Therefore, if the product of thediameter of the exhaust gas outlet duct 2 or the width of the hearth 12and the flow rate of the combustion gas is 7.2 m²/sec or more, theReynolds number becomes 30,000 or larger. Further, if the product of thediameter of the exhaust gas outlet duct 2 or width of the hearth 12 andthe flow rate of the combustion gas is 12 m²/sec or more, the Reynoldsnumber becomes more than 50,000. As explained above, to obtain therequired Reynolds number, the method of controlling the gas flow rateconsidering the width of the hearth and the diameter of the outlet ductis effective.

[0061] Note that needless to say, after this state is realized, theconcentration of carbon monoxide is kept from increasing again.

[0062] Next, it is important to cool the combustion gas from 800° C. to300° C. rapidly. The objective is to shorten the time when the gas is inthe temperature band of 300 to 600° C. where dioxins are resynthesized.Therefore, the inventors analyzed the time required for cooling. FIG. 5shows the results of experiments. If the cooling time is over 6 seconds,the concentration of dioxins becomes more than 0.1 ng-TEQ/Nm³. That is,it is important that the cooling time from 800° C. to 300° C. be madenot longer than 6 seconds. Therefore, it is important that the insidevolume of the cooling portion from 800° C. to 300° C. of the exhaust gastreatment apparatus be not more than 6 times the amount of flow ofcombustion gas per second.

[0063] If this cooling speed is kept, the combustion gas can be cooledby any method. In the example of FIG. 1, use was made of a combinationof a waste heat boiler, water sprayer, and air preheating heat exchanger(heat exchanger between combustion gas and air). This method is anoptimal method for reclaiming waste heat to the maximum and accuratelycontrolling the temperature of the combustion gas. If simplifying thefacility, however, it is also possible to omit the waste heat boiler andprovide just a combination of a water sprayer and air preheater. In thiscase, the amount of waste heat reclaimed becomes about one-half that ofthe facility shown in FIG. 1. Further, when not reclaiming waste heat atall, it is also possible to cool the combustion gas to 300° C. or lowerby just a water sprayer.

[0064] In this way, for efficient cooling, it is preferable to preventdeposition of dust on the cooler or the heat reclaimer (heat exchanger)of the exhaust gas treatment apparatus. As explained below in detail, itis preferable to make the total number of moles A of the zinc and lead,the total number of moles B of the sodium and potassium, and the totalnumber of moles C of the chlorine and fluorine contained in the dust ofthe combustion gas generated in the furnace satisfy the relationship of(C−B)A<0.36 so as to prevent a decrease in the heat exchange and coolingefficiencies due to deposition on the cooler or heat reclaimer (heatexchanger) and the narrowing of the exhaust gas passage and clogging offlow due to deposition of dust in the duct. In addition, it ispreferable to make the content of the compounds of alkali metals (sodiumor potassium) and halogen elements (chlorine or fluorine) contained inthe dust of the gas generated in the furnace 35 mass % or less so as tosuppress the deposition of dust. Further, preferably by cooling thetemperature of the gas generated in the furnace from more 800° C. orhigher to 550° C. or lower within 5 seconds, it is possible to suppressthe deposition of dust on the exhaust gas treatment apparatus andeffectively suppress a decrease in the cooling efficiency.

[0065] However to suppress the generation of dioxins, the exhaust gas israpidly cooled in the exhaust gas treatment apparatus, and at this time,heat exchange and the reclamation of heat are extremely important fromthe viewpoint of improving the heat efficiency in the operation.

[0066] However as explained above, due to the various problems arisingdue to the large amount of dust contained in this high temperatureexhaust gas stick to the heat storage boiler, air preheating type heatexchanger, or ether heat reclaimer or cooler in the exhaust gastreatment apparatus, this is not efficient. These problems give aneffect not only on the heat reclamation efficiency, but also the coolingefficiency of the exhaust gas for suppressing dioxins. The inventorsfurther studied these points.

[0067] The combustion exhaust gas includes as dust componentscarried-over feedstock such as unreduced iron oxide etc., substanceswhich easily vaporize at a high temperature such as alkali metals andhalogen elements, and metals such as zinc, lead, and others whichvaporize after being reduced. Among these substances, alkali halogensalts and zinc chloride or lead chloride themselves have melting pointsof 700 to 900° C. However, the inventors discovered that if the ratio ofthe alkali halogen salts and zinc chloride and lead chloride in the dustin the exhaust gas rises, these substances melt together and lower themelting point to 550 to 600° C. Further, they discovered that the dustwith these conditions easily deposits in the path of the exhaust gas,that is, the passage of the exhaust gas from the exhaust gas duct of thefurnace through the exhaust gas treatment apparatus to the atmosphere.

[0068] The clumps of the dust deposited in the path of the exhaust gaswere analyzed. As a result, it was discovered that when the amount ofchlorides and fluorides increases, these components in the dust stick ata temperature of 450 to 600° C. or higher and in particular when theratio of the halides of zinc and lead to oxides of zinc and lead rises,the deposition becomes intense. Further, the inventors investigated thedeposition of dust at the temperature of 550° C. where the heatexchanger etc. operated normally. As the result of the investigation, itwas discovered that if the ratio of the halides of zinc and lead in thedust is 18% or less, converted to moles, with respect to the total ofthe oxides and halides of zinc and lead, the stickiness is relativelyweak and the dust can be easily removed. Even under this condition,however, it was learned that when the mass ratio of alkali halogensalts, that is, compounds of alkali metals (sodium or potassium) andhalogen elements (chlorine or fluorine) exceeds 35% of the mass of thedust as a whole, the deposition increased somewhat. Therefore, thepercentage of compounds of alkali metals and halogen elements containedin the dust was made not more than 35 mass %.

[0069] Due to this, in the present invention, the ratio of the elementsin the dust is controlled so that the halides of the zinc and lead inthe dust becomes not more than 22% in molar ratio with respect to theoxides of zinc and lead. The molar ratio of the zinc, lead, sodium,potassium chlorine and fluorine in the dust is controlled.

[0070] The control of the molar ratio is that the number of moles ofhalogen elements not fully reacting with alkaline, that is, (number ofmoles of chlorine+number of moles of fluorine)−number of moles ofsodium+number of moles of potassium), is made not more than 18% of thetotal number of moles of zinc and number of moles of lead. Note thatzinc and lead form bivalent anions, so halogen elements, which aremonovalent cations, are corrected by ion value. As the indicator foroperation, [(number of moles of chlorine+number of moles offluorine)−(number of moles of sodium+number of moles ofpotassium)]/(number of moles of zinc+number of moles of lead);(indicator 1) is used. In actual operation, the condition is expressedas indicator 1 <0.36. That is, when the total number of moles of zincand lead contained in the dust is A, the total number of moles of sodiumand potassium is B, and the total number of moles of chlorine andfluorine is c, (C−B)/A<0.36.

[0071] To control the ingredients of the dust accurately, it isnecessary to control the ingredients of the feedstock. In the rotaryhearth method, the rate of transition of the zinc and lead from thefeedstock to the exhaust gas is at least 90%, the rate of transition ofthe alkali metals is about 70%, and the rate of transition of thehalogen elements is about 80%. The ratios of the zinc, lead, alkalimetals, and halogen elements in the feedstock are determined consideringthese transition rates. If expressing this by a formula, the result is[0.8 (number of moles of chlorine+number of moles of fluorine)−0.7(number of moles of sodium+number of moles of potassium)]/[0.9 to 1.0(number of moles of zinc+number of moles of lead)], so the furnace isoperated using the simple formula [0.8 (number of moles ofchlorine+number of moles of fluorine)−0.7 (number of moles ofsodium+number of moles of potassium)]/(number of moles of zinc+number ofmoles of lead); (indicator 2) as an indicator of control of mixing ofthe feedstock. In actual operation, the condition is expressed asindicator 2 <0.36. That is, when the total number of moles of zinc andlead contained in the feedstock is A, the total number of moles ofsodium and potassium is B′, and the total number of moles of chlorineand fluorine is C′, (0.8C′−0.7B′)/A′<0.36.

[0072] To control the ingredients of the dust accurately, it isnecessary to control the ingredients of the feedstock. In the rotaryhearth method, the rate of transition of the zinc and lead from thefeedstock to the exhaust gas is at least 90%, the rate of transition ofthe alkali metals is about 70%, and the rate of transition of thehalogen elements is about 80%. The ratios of the zinc, lead, alkalimetals, and halogen elements in the feedstock are determined consideringthese transition rates. If expressing this by a formula, the result is[0.8 (number of moles of chlorine+number of moles of fluorine)−0.7(number of moles of sodium+number of moles of chlorine and fluorine isc, (C−B)/A<0.36.

[0073] Exhaust gas containing dust adjusted under these conditions isintroduced to the exhaust gas treatment apparatus from the exhaust gasoutlet duct 2. The temperature of the exhaust gas at this portion ismade 800° C. or higher. If it is lower than 800° C., the chlorides andfluorides that enter the exhaust gas outlet duct 2 immediatelyprecipitate and cause clogging near the inlet of the exhaust gas outletduct 2. Further, as explained above, the temperature is made 800° C. orhigher from the viewpoint of suppression of the generation of dioxins.

[0074] Next, the exhaust gas is introduced into the waste heat boiler 3where the heat is reclaimed, then the exhaust gas is rapidly cooled to550° C. or lower. If the temperature of the exhaust gas at the outlet ofthe waste heat boiler 3 becomes higher than 550° C. the exhaust gas isfurther cooled by the water spray cooler 4 to drop the temperature ofthe gas to 550° C. or lower. It is important to rapidly cool the exhaustgas from the waste heat boiler 3 to the water spray cooler 4 and shortenthe time of the chlorides and fluorides being in the fused state. Theinventors found that with the dust shown in the present invention, noserious deposition of dust occurred at the rapid cooling portion ifreducing the temperature to 550° C. or lower within 5 seconds.Therefore, performing the cooling treatment as a whole by a water spraytype gas cooler etc. is also effective.

[0075] The exhaust gas is further heat exchanged with air to reclaim theheat and cooled to not higher than 300° C., preferably about 200° C.Note that as mentioned above, from the viewpoint of suppressingresynthesis of dioxides, the cooling from 800° C. to not higher than300° C. is preferably performed within 6 seconds. The gas temperature atthe inlet part of the heat exchanger 5 is an important factor in theoperation. That is, even with the dust composition of the scope of thepresent invention, if the temperature of the exhaust gas in the heatexchanger is too high, the sodium chloride, zinc chloride, etc. whichdid not finish solidifying deposits on the metal surfaces of the heatexchanger in a liquid state and cause the problem of blockage or metalcorrosion. To prevent fouling of the heat transmission surfaces due todeposition of dust so as to reclaim waste heat efficiently and toprevent deposition of the dust, it is preferable to install a dustremover at the heat exchanger 5. There are various types of dustremovers. Among them, generally used are a soot blow type which blowshigh pressure gas or steam and the impact type. The type of the heatexchanger is also an important requirement for prevention of thedeposition of dust. A heat exchanger resistant to the deposition of dustis of a type passing air inside the tubes and exhaust gas outside thetubes to remove dust by soot blowing outside of the tubes or a typeprovided with a large number of parallel plates through which air andexhaust gas alternately flow and removing the deposited dust by ascraper.

[0076] The air heated by the heat exchange at this time is used as airfor combustion in the rotary furnace 1 or hot air for predrying of thepellets.

[0077] The thus heat exchanged and cooled combustion exhaust gas is thenpassed through a duct 6 before the dust collector and cleaned of dust bythe dust collector 7.

[0078] The dust collector 7 preferably is of a bag filter type notresynthesizing dioxins inside it. A “bag filter” is a device having alarge number of filter cloths comprised of cloths with fine intervalsbetween fibers formed into bag shapes and removes dust by passing gascontaining dust through the filter cloths. The operating temperature ofthe bag filter is preferably at least 150° C. The reason is that ifbelow 150° C., the temperature drops below the temperature of acidcondensation and the facility is easily corroded. Further, in view ofthe problem of the heat resistance of the filter cloths, a temperatureof not higher than 250° C. is preferable. With a rotary hearth, whenreducing the metal oxide, oxides or chlorides of zinc, lead, alkalimetals, etc. are exhausted into the combustion gas. These are submicronsized powders. With a cyclone etc., sufficient dust collection is notpossible. Further, an electric dust collector has an insufficient dustcollecting capability and induces resynthesizing reaction of dioxinsinside it, so is not suitable for this process. A bag filter type dustcollector is the best.

[0079] However, when cooling a large part of the sensible heat of thecombustion gas by spraying water, the concentration of water vapor inthe combustion gas becomes too high and a wetting at the bag filterarises, so a wet type dust collector is preferable. In particular, whenthe moisture in the combustion gas exceeds 30 mass %, the bag filterbecomes extremely wet. Due to this, clogging and corrosion of the filteroccur. To keep the moisture in the combustion gas not exceeding 30 mass%, it is necessary to make the amount of water added during the sprayingnot more than 400 kg per 1 Nm³ of combustion gas. When spraying morethan this amount, a venturi scrubber or other wet type dust collector isused.

[0080] Finally, the combustion exhaust gas after removal of the dust wasdischarged into the atmosphere from the smokestack 9 by the drive forceof an induction fan 8.

[0081] As explained above, by operating the reducing furnace of a rotaryhearth method by the method of the present invention, it is possible tokeep the production of dioxins to a minimum and operate in a manner notcausing much environmental pollution, it is possible to suppress thedeposition of dust on the exhaust gas treatment apparatus andefficiently exchange heat and cool, and it is possible to realize anoperation with a high heat efficiency, even when reducing metal usingbyproducts in the metal industry as a feedstock.

EXAMPLES Example 1 to Example 3 and Comparative Example 1

[0082] Using the reducing furnace of the rotary hearth method describedin FIG. 1, powder coke was mixed in fine iron ore, electric arc furnacedust, and particulate rolling scale to produce 10 to 20 mm greenpellets. They were reduced in the rotary furnace. The rotary furnace hada center diameter of the hearth of 17 m and a hearth width of 4 m andhad the capacity to produce 15 tons/h of reduced iron. The length of thereducing zone 19 of this rotating furnace was 35 m, while the length ofthe heating zone 18 was 18 m. The results of operation by this facilityare shown in Table 1. Further, the combustion gas was generated at arate of 27,000 to 30,000 Nm³/h in Examples 1 and 2 and the comparativeexample, while was 17,000 Nm³/h in Example 3.

[0083] In Example 1, the temperature of the reducing zone of the rotaryfurnace was made 1270° C. and the molar ratio of carbon monoxide tocarbon dioxide was made 0.55. The green pellets were reduced in thereducing zone 19 for 8 minutes. This combustion gas flowed to theheating zone 18 where it was gradually cooled. When it reached the 7 mpoint to the exhaust gas outlet duct, the combustion gas was 1030° C.,the concentration of carbon monoxide was 88 ppm by volume ratio, and theconcentration of oxygen was 1.1 vol %. The temperature of the combustiongas at the inside of the exhaust gas outlet duct 2 was 980° C., theconcentration of carbon monoxide was 69 ppm by volume ratio, and theconcentration of oxygen was 1.3 vol %.

[0084] The flow rate of the combustion gas inside the furnace at aportion 5 m to the exhaust gas outlet duct 2 was 5.5 m/sec and insidethe exhaust gas outlet duct 2 of a length of 6 m was 6.1 m/sec. Thewidth of the hearth 12 at the heating zone inside the furnace was 4 m,the product of the hearth width and combustion gas flow rate was 22m²/sec, and the Reynolds number was 100,000. The diameter of the exhaustgas outlet duct 2 was 2.4 m, the product of the duct diameter and thecombustion gas flow rate was 14.6 m²/sec, and the Reynolds number was70,000. Therefore, the time during which the conditions of thetemperature and ingredients of the combustion gas of the presentinvention were satisfied was 0.9 second inside the heating zone in thefurnace, and it was 1.0 second inside the exhaust gas outlet duct 2. Thetotal was 1.9 seconds.

[0085] Further, the combustion gas was cooled from 980° C. to 210° C.over 5.7 seconds in the interval from the waste heat boiler 3 to the airpreheater 5. Next, the combustion exhaust gas was cleaned of the dust bythe bag filter type collector 7 and discharged into the atmosphere. Theconcentration of dioxins in the combustion exhaust gas at that time was0.05 ng-TEQ/Nm³, that is, a small load on the environment.

[0086] Example 2 shows the results of operation under even betterconditions. The temperature of the combustion gas of the exhaust gasoutlet duct 2 was 1105° C., while the concentration of carbon monoxidewas 45 ppm in volume ratio. The Reynolds number of the exhaust gasoutlet duct 2 was 72,000. The time while the combustion gas was 800° C.or higher was 0.8 second or within the conditions of the presentinvention. Further, the time for cooling from 800° C. to 300° C. was 4.7seconds. As a result, the concentration of the dioxins in the combustiongas was an extremely good 0.02 ng-TEQ/Nm³.

[0087] Example 3 shows the results of operation under conditions with asmaller Reynolds number. In this operation, the conditions were that ofa reduced reducing speed of the pellets and low flow rate of thecombustion gas. In the furnace, the concentration of carbon monoxide ofthe combustion gas was not more than 300 ppm in volume ratio and thecombustion gas was treated as to give 130 ppm of carbon monoxideconcentration at the inlet of the exhaust gas outlet duct 2. In thisoperation, the Reynolds number of the exhaust gas outlet duct 2 was42,000. The temperature of this combustion gas was 910° C. and the timeit was above 800° C. was 1.4 seconds or within the conditions of thepresent invention. Further, the time for cooling from 800° C. to 300° C.was 4.9 seconds. As a result, the concentration of dioxins in thecombustion gas was somewhat high, but a good 0.101 ng-TEQ/Nm³. Thereduction rate of the pellets in the operations of these examples was agood 85% or so, on the other hand, in the comparative example, thoughgreen pellets of the same feedstock as the examples were reduced by theabove rotary furnace 1, the concentration of dioxins in the combustiongas was high since the operating conditions of the present inventionwere not performed. In the comparative example, the temperature of thereducing zone and the ingredients of the combustion gas were similar tothose of Example 1, but the temperature of the heating zone and theingredients of the combustion gas were different. Among the conditions,only the Reynolds number was a high 60,000. In the heating zone,however, the temperature of the combustion gas was 760° C. at theminimum. Further, the atmosphere was oxygen deficient, the concentrationof oxygen was about 0 vol %, and the concentration of carbon monoxidewas 800 ppm by volume ratio. The concentration of carbon monoxide was500 ppm even inside the exhaust gas outlet duct 2.

[0088] As a result, the concentration of dioxins in the combustion gasat the smokestack 9 was a relatively high 0.52 ng-TEQ/Nm³. That is, inthe comparative example, it was not possible to suitably control thetemperature and ingredients of the combustion gas, so it was notpossible to reduce the generation of dioxins. TABLE 1 Ex. 1 Ex. 2 Ex. 3Comp. Ex. 1 Reducingzone Temperature 1270° C. 1300° C. 1280° C. 1270° C.CO/CO₂ ratio 0.55 0.41 0.45 0.55 (molar ratio) Exhaust gas Temperature 980° C. or 1150° C.  910° C.  780° C. outlet higher duct CO concentra-69 to 88 45 ppm 135 ppm 500 ppm vicinity tion (vol %) ppm Reynolds70,000 or 72,000 42,000 60,000 number larger Holding time 1.9 sec. 0.8sec. 1.5 sec. — Subsequent 5.7 sec. 4.7 sec. 4.9 sec. — cooling time orshorter or shorter (800° C.→ 300° C.) Pellet reduction rate 85% 84% 87%83% concentration of dioxins in 0.05 0.02 0.101 0.52 exhaust gas(ng-TEQ/Nm³)

Example 4 to Example 6 and Comparative Example 2

[0089]FIG. 1 shows an exhaust gas treatment facility of a reducingfurnace using a rotary hearth used in Example 4 to Example 6 andComparative Example 2 of this method of operation of the presentinvention. This reducing furnace reduces the green pellets of thefeedstock at a rate of 18 tons per hour. A 1100° C. exhaust gas isgenerated at a rate of 45,000 Nm³/h. Table 2 shows the operatingconditions and results of examples and a comparative example.

[0090] Example 4 is an example of an operation using iron ore andconverter dust as iron sources and fine coke as a reducing agent. Thisfeedstock has relatively little impurities. The indicator 2 of thefeedstock ingredient was 0.18, that is, within the range of the presentinvention. As a result the indicator 1 of the dust ingredient was also0.17 and the ratio of the alkali halogen salts was a low 20 mass %. Theexhaust gas treatment conditions were that an exhaust gas temperature atthe inlet of the exhaust gas outlet duct 2 was of a high 970° C., andthe temperature of the inlet of the heat exchanger 5 was a low 480° C.As a result, there was no deposition of dust in the exhaust gastreatment apparatus at all.

[0091] Next, Example 5 is an example of operation using blast furnacegas dust with a relatively large amount of impurities. While the amountof impurities was large, the indicator 2 of the feedstock ingredientswas 0.16 or substantially the same as in Example 4. As a result, theindicator 1 of the dust ingredients was 0.13. Compared with Example 1,since the feedstock contained lime, the conditions became ones wherehalogens easily remained in the reduced product. Compared with theindicator 2, it is believed that the indicator 1 became smaller. Theratio of the alkali halogen salts was also a low 21 mass %. In thisoperation as well the exhaust gas treatment conditions were that anexhaust gas temperature at the inlet of the exhaust gas outlet duct 2was of a high 1000° C. and an exhaust gas temperature at the inlet ofthe heat exchanger 5 was of a low 460° C. As a result there was nodeposition of dust in the exhaust gas treatment system at all.

[0092] An Example 6 has ingredients of the feedstock and dust within therange of the present invention. While this follows a method of thepresent invention, this is the result of operation under conditionswhere the exhaust gas temperature of the exhaust gas treatment apparatusis not good. Note that the feedstock used was the same as in Example 5.The indicator 1 and indicator 2 were the same as in Example 5. However,the exhaust gas temperature at the inlet of the exhaust gas outlet duct2 was a low 760° C., while the exhaust gas temperature at the inlet ofthe heat exchanger was a high 570° C. As a result, the problem arose ofslight deposition of the dust in the exhaust gas treatment apparatus.When inspected after operation for three months, a deposit was observedat the inlet of the exhaust gas outlet duct 2. Further, the inlet of theheat exchanger 5 gradually became clogged. Thus cleaning was required intwo months.

[0093] The results of operation in adjustment of the feedstock, exhaustgas temperature, and dust control are shown in Comparative Example 2. InComparative Example 1, since large amounts of chlorine and otherfeedstock were used, the indicator 2 of the ingredients was 0.48. As aresult, the indicator 1 of the dust was 0.51. Further, the ratio of thealkali halogen salts was also a high 30 mass %. As a result, while thetemperature condition of the exhaust gas was good, there was a depositat the inlet of the exhaust gas outlet duct 2 which required cleaningafter one month. Further, the inlet of the heat exchanger 5 becameclogged fast and operation no longer was possible after 2 weeks. In thisway, when outside the conditions of the present invention, long term,stable operation cannot be continued.

[0094] On the other hand, by using the present invention, it waspossible to continue stable operation for a long period and exchangeheat well. Therefore it was possible to reduce metal oxides at a highenergy efficiency and possible to reduce the cost of metal productiongreatly. TABLE 2 Ex. 5 Ex. 6 Comp. Ex. 2 Ex. 4 Blast furnace Blastfurnace Pickling Iron ore gas dust gas dust sludge Converter ConverterConverter Converter dust dust dust dust Ingredient fine coke fine cokefine coke fine coke Feedstock Zinc (mass %) 0.44 1.11 1.11 0.88 Lead(mass %) 0.08 0.07 0.07 0.09 Sodium (mass %) 0.11 0.09 0.09 0.19Potassium 0.09 0.14 0.14 0.11 (mass %) Chlorine 0.11 0.19 0.19 0.47(mass %) Fluorine 0.09 0.09 0.09 0.09 (mass %) Indicator 2 0.18 0.160.16 0.48 Dust Zinc (mass %) 39 53 53 37 Lead (mass %) 3 4 4 4 Sodium{mass %) 4 4 4 6 Potassium 5 5 5 4 {mass %} Chlorine 7 9 9 16 (mass %)Fluorine 4 3 3 4 {mass %} Indicator 1 0.17 0.13 0.13 0.51 Alkali halogen20 21 21 30 salts (mass%) Exhaust gas temperature Exhaust gas 970 1000760 1000 introduction duct (° C.) 480 460 570 460 Heat exchanger inlet(° C.) Exhaust gas None None Some Large introduction duct cloggingdeposition deposition Heat exchanger None None Clean after 2 Clogged in2 inlet clogging months weeks

INDUSTRIAL APPLICABILITY

[0095] According to the present invention, in the operation of areducing rotary hearth furnace or other firing reducing furnace, it ispossible to suppress the production of dioxins in combustion exhaust gasto ½ to {fraction (1/10)} that of the past. Further, it is possible tosuppress blockage of the path of the exhaust gas in the exhaust gastreatment apparatus due to deposition of the dust, to reclaim waste heatfrom the high temperature exhaust gas efficiently, and to raise the heatefficiency of the firing reducing furnace. Due to this, it is possibleto reduce dust, sludge, scale, and other by-products produced from themetal industry effectively, possible to perform the operation at a highoperating rate, and possible to reduce the production costs of reducedmetal.

1. A method of operation of a reducing rotary hearth furnacecharacterized by making the combustion gas generated inside a rotaryhearth reducing furnace a temperature of 800° C. or higher at least atone of the inside of an exhaust gas outlet duct of said reducing furnaceand the vicinity of the exhaust gas outlet duct then rapidly cooling it,collecting dust, then discharging the result.
 2. A method of operationof a reducing rotary hearth furnace characterized by making thecombustion gas generated inside a reducing rotary hearth furnace atemperature of 800° C. or higher at least at one of the inside of anexhaust gas outlet duct and the vicinity of the exhaust gas outlet duct,making a concentration of carbon monoxide not more than 200 ppm byvolume ratio, making the gas a well developed turbulent state, thenrapidly cooling said combustion gas.
 3. A method of operation of areducing rotary hearth furnace characterized by making the combustiongas generated inside a reducing rotary hearth furnace a temperature of800° C. or higher for at least 0.9 second at least at one of the insideof an exhaust gas outlet duct and the vicinity of the exhaust gas outletduct, making a concentration of carbon monoxide not more than 200 ppm byvolume ratio, making the gas a turbulent state of a Reynolds number ofat least 30,000, then rapidly cooling said combustion gas.
 4. A methodof operation of a reducing rotary hearth furnace characterized by makingthe combustion gas generated inside a rotary bed type reduction furnacea temperature of 800° C. or higher for at least 0.6 second at least atone of the inside of an exhaust gas outlet duct and the vicinity of theexhaust gas outlet duct, making a concentration of carbon monoxide notmore than 60 ppm by volume ratio, making the gas a turbulent state of aReynolds number of at least 50,000, then rapidly cooling said combustiongas.
 5. A method of operation of a reducing rotary hearth furnace as setforth in any one of claims 1 to 4, characterized in that the combustiongas generated in said furnace has a temperature inside the furnace of atleast 1200° C. and a molar ratio of carbon monoxide to carbon dioxide ofat least 0.1.
 6. A method of operation of a reducing rotary hearthfurnace as set forth in any one of claims 1 to 5, characterized in thatthe combustion gas generated inside is cooled from 800° C. or higher tonot more than 300° C. within 6 seconds.
 7. A method of operation of areducing rotary hearth furnace as set forth in claim 6, characterized inthat the combustion gas generated inside is cooled in an exhaust gastreatment apparatus provided with anyone of a waste heat boiler, watersprayer, and heat exchanger with air alone or in combination.
 8. Amethod of operation of a reducing rotary hearth furnace as set forth inany one of claims 1 to 7, characterized in that the total number ofmoles A of zinc and lead, the total number of moles B of sodium andpotassium, and the total number of moles C of chlorine and fluorinecontained in the dust of the combustion gas generated inside satisfy therelationship of (C−B)/A<0.36.
 9. A method of operation of a reducingrotary hearth furnace as set forth in any one of claims 1 to 8,characterized in that the percent content of compounds of sodium orpotassium and chlorine or fluorine contained in the dust of thecombustion gas generated inside is not more than 35 mass %.
 10. A methodof operation of a reducing rotary hearth furnace as set forth in claim 8or 9, characterized in that the temperature of the gas generated insideis cooled from 800° C. or higher to 550° C. or lower within 5 seconds.11. A method of operation of reducing rotary furnace wherein acombustion exhaust gas is cooled in a gas treatment apparatus providedwith at least a preheating type heat exchanger preheating air by heatexchanging, said method characterized in that the total number of molesA of zinc and lead, the total number of moles B of sodium and potassium,and the total number of moles of C of chlorine and fluorine contained inthe dust of the combustion gas generated inside satisfy the relationshipof (C−B)/A<0.36.
 12. A method of operation of a reducing rotary hearthfurnace as set forth in any one of claims 1 to 15, characterized in thatthe content ratio of compounds of sodium or potassium and chlorine orfluorine contained in the dust of the combustion gas generated inside isnot more than 35 mass %.
 13. A method of operation of a reducing rotaryhearth furnace as set forth in claim 11 or 12, characterized in that thetemperature of the exhaust gas at the inlet of the exhaust gas outletduct of said reducing furnace is 800° C. or higher and temperature ofthe exhaust gas at the inlet of the air preheating heat exchanger ofsaid exhaust gas treatment apparatus is 550° C. or lower.
 14. A methodof operation of a reducing rotary hearth furnace as set forth in any oneof claims 1 to 13, characterized by cooling the temperature of saidexhaust gas from 800° C. or higher to 550° C. or lower within 5 seconds.15. A method of operation of a reducing rotary hearth furnace for firingand reducing a feedstock obtained by shaping a powder containing a metaloxide and carbon and cooling the combustion exhaust gas in a rotaryhearth reducing furnace having an exhaust gas treatment apparatusprovided with at least an air preheating type heat exchanger, saidmethod of operation of a reducing rotary hearth furnace characterized inthat the relationship of (0.8C′−0.7B′)/A′<0.36 stands among the totalnumber of moles A′ of zinc and lead, the total number of moles B′ ofsodium and potassium, and the total number of moles C′ of chlorine andfluorine in the feedstock, the temperature of the exhaust gas at theexhaust gas outlet duct of said reducing furnace is made 800° C. orhigher, and the temperature of the exhaust gas at the inlet of said airpreheating type heat exchanger is made at least 550° C.
 16. A method ofoperation of a reducing rotary hearth furnace as set forth in any one ofclaims 1 to 15, characterized in that the cooled combustion gasgenerated inside the furnace is cleaned of dust by a bag filter of anexhaust gas treatment apparatus.
 17. A reducing rotary hearth furnacefacility characterized by being provided with a reducing furnace for ametal oxide having a rotary hearth and an exhaust gas treatmentapparatus comprised of a cooler comprised of a waste heat boiler, watersprayer, or air preheating heat exchanger alone or in combination and adust collector and by connecting said reducing furnace and said exhaustgas treatment apparatus by a duct having a length of at least 0.9 secondconverted to time of passage of the combustion gas generated inside saidreducing furnace and a product of the inside diameter and flow rate ofgas inside at least 7.2 m²/sec.
 18. A reducing rotary hearth furnacefacility characterized by being provided with a reducing furnace for ametal oxide having a rotary hearth and an exhaust gas treatmentapparatus comprised of a cooler comprised of a waste heat boiler, watersprayer, or air preheating heat exchanger alone or in combination and adust collector, and by connecting said reducing furnace and said exhaustgas treatment apparatus by a duct having a length of at least 0.6 secondconverted to time of passage of the combustion gas generated inside saidreducing furnace and a product of the inside diameter and flow rate ofgas inside at least 12 m²/sec.
 19. A reducing rotary hearth furnacefacility as set forth in claim 17 or 18, characterized in that thecooler in said exhaust gas treatment apparatus has an inside capacity ofthe portion cooling the combustion gas from more than 800° C. to notmore than 300° C. or not more than six times the amount of flow ofcombustion gas per second.
 20. A reducing rotary hearth furnace facilitycharacterized by being provided with a reducing furnace for a metaloxide having a rotary hearth and an exhaust gas treatment apparatuscomprised of a cooler comprised of a waste heat boiler, water sprayer,or air preheating heat exchanger alone or in combination and a dustcollector and by being provided with a means for removing dust depositedon said air preheating heat exchanger of said exhaust gas treatmentapparatus of at least one of an impact/vibration imparting device, gasblower, and scraper.